The invention relates to a circuit arrangement for operating discharge lamps with alternating current, especially high and extra-high-pressure discharge lamps as used in devices for projecting images. The invention deals with the problem of flickering phenomena which are caused by an arc perturbation of the discharge lamps after a polarity change of the lamp current. In particular, the invention deals with problems which are associated with detection of changes of the electrodes of the discharge lamps.
There is in the operation of discharge lamps, which will also be called lamp for short in the text which follows, the phenomenon of the growth of electrode spikes. Material which is evaporated from the electrodes at one point is deposited again on the electrode at preferred points and leads to the formation of electrode spikes. These electrode spikes initially have the advantage that the plasma arc of the arc discharge generated in the lamp finds a stable root point of the electrode and does not jump between several root points. This jumping of the discharge point is also called arc jumping and manifests itself in a flickering of the lamp. This is particularly disturbing when the light of the lamp is used for projecting images.
The root point of the arc only forms on an electrode which currently acts as a cathode. The root of the arc on the anode is two dimensional. Arc jumping is therefore a wide-spread problem in lamps which are operated with alternating current since, with each polarity change, the arc must find a root point on the electrode changing from the anode to the cathode. The electrode spikes described above form a preferred root point for the arc and thus reduce arc jumping.
However, problems can also occur due to the electrode spikes. Under unfavorable conditions, several electrode spikes can form. It can then happen that the arc root jumps between the various electrode spikes.
In the document EP 1 624 733 A2 (Suzuki), this problem is solved by reducing the operating frequency, i.e. the frequency of alternating current with which the lamp is operated, for a limited time. This method works due to the fact that an electrode heats whilst it is acting as an anode and cools down whilst it is acting as a cathode. This provides a temperature fluctuation with a variation in time which corresponds to the operating frequency. At high frequencies, a mean temperature occurs due to the thermal capacity of the electrodes. In so-called square-wave operation, the lamps are operated with a square-wave current having a frequency of typically 50 to 5000 Hz. Depending on the design of the lamp, a noticeable temperature modulation of the electrodes can already occur at 50 Hz. With a strong temperature modulation, the electrode reaches temperatures at which the redundant electrode spikes are melted off.
Video projectors frequently need a light source which exhibits a sequence of different colors in time. As is described in the document U.S. Pat. No. 5,917,558 (Stanton), this can be achieved by means of a rotating color wheel which filters alternating colors out of the light of the lamp. The periods during which the light assumes a certain color do not need to be mandatorily identical. Instead, a desired color temperature produced for the projected light can be set via the ratio of these periods with respect to one another.
The lamp is usually operated with a square-wave lamp current. The above-mentioned operating frequency is understood to be the reciprocal of the period of the square-wave lamp current. In the prior art, the lamp current is generated from a direct-current source with the aid of a commutating device. The commutating device usually consists of electronic switches which commutate the polarity of the direct-current source at the rate of the square-wave lamp current. In practice, overshoots cannot be avoided completely during the commutation. For this reason, to mask out the overshoots, the time at which a commutation is to take place is combined with the time at which the color of the light changes, in the prior art. For this purpose, a sync signal is provided which exhibits a sync pulse in synchronism with the abovementioned color wheel. With the aid of the sync signal the color change and the commutation of the lamp current are synchronized.
In general, electrode spikes do not grow in the same manner on both electrodes. The reason for this can be that the installation conditions of the lamp influence the heat balance of the electrodes differently. It is especially when a lamp is not operated horizontally that this, as a rule, leads to a higher temperature of the more highly placed electrode. If spikes grow several times on an electrode, a melting-off of the electrodes is proposed in the abovementioned prior art in order to avoid flickering phenomena. However, melting-off of both electrodes can damage the electrode having only one electrode spike. This one electrode spike is possibly melted off even though it does not cause any flickering phenomena.